(1) The strict meaning of this prefix according to chemical nomenclature is “one methyl group on the next-to-last carbon atom, and no other branches”. In the plasticizer field, the prefix is used to denote an isomer of a compound, specifically an isomer having a single, simple branching, not limited to methyl, at the end of a straight chain. (2) Equal, same, or constant, as in isothermal, at constant temperature.